Semiconductor laser diodes are frequently used as radiation sources in optical fiber communications, as well as in other fields of technology. In many cases, the laser is operated in a pulse mode, e.g., having a two-level output, with the levels corresponding to logical one and logical zero, respectively. For instance, digital laser transmitters for optical communications frequently are driven by a high frequency modulation current superimposed on a dc bias current. The magnitude of the dc bias current is typically chosen so as to bias the laser near threshold. This practice results in a laser output such that the logic zero corresponds to no, or essentially no, radiation emitted from the laser, and the logic one level to a finite radiation intensity emitted from the laser. The dc bias current serves several purposes, but foremost among these are the functions of minimizing turn-on delay and minimizing the amount of current that must be switched at the data rate.
Because of possible drift in laser threshold due to aging and temperature variations, it is typically necessary to stabilize the bias current using feedback. The prior art knows several methods for achieving stabilization. In systems applications, it has been the practice to use optical feedback, with an external photodetector either at the backside of the laser or at the front, using an optical fiber tap. See, for instance, pp. 182-186 of the article by P. W. Shumate and M. DiDomenico, "Lightwave Transmitters", of Semiconductor Devices for Optical Communications, H. Kressel, editor, second edition, Springer-Verlag, 1982. It is to be noted that such prior art stabilization schemes hold the light output of the laser constant, but do not necessarily stabilize the bias current at the desired level. This is a significant shortcoming, since several transmitter parameters, in addition to the extinction ratio, depend strongly on whether the laser is biased above, at, or below threshold. For example, when the laser is biased below threshold, extra turn-on delay typically occurs, leading to data pattern-dependent modulation errors in high speed systems. Also, in single-frequency laser transmitters, frequency shifting may occur as the modulation current drives the laser through threshold. And furthermore, in single loop stabilization systems, the efficacy of feedback in controlling laser variations is typically reduced when bias is sub-threshold. For a discussion of these points, see, for instance, G. Arnold and P. Russer, Applied Physics, Volume 14, November 1977, pp. 255-268; R. A. Linke, Electronics Letters, Volume 20, No. 11, May 1984, pp. 472-474; and R. G. Swartz and B. A. Wooley, Bell System Technical Journal, Volume 62, No. 7, Part 1, September 1983, pp. 1923-1936. On the other hand, when the laser is biased above threshold, reduced extinction ratio results. Also, when the laser is heated, or it ages, the threshold tends to increase so that a bias level initially above threshold can drop below it.
Based on these and other considerations, it would be advantageous to have available a method for stabilizing of the bias point at or near threshold.
As is well known, the dynamical resistance of a laser, i.e., the first derivative of its V/I characteristic, typically has a "kink" at threshold. This is illustrated in FIG. 1, which shows the quantity dV/dI as a function of the forward current through a semiconductor laser. Section 10 of the curve is the dynamical resistance below lasing threshold, and Section 11 above lasing threshold, with 12 referring to the "kink" in the dynamical resistance which coincides with the lasing threshold. See, for instance, R. W. Dixon, Bell System Technical Journal, Volume 55, No. 7, September 1976, pp. 973-980.
First and second derivative measurements of semiconductor laser I/V characteristics have been reported. These measurements were typically accomplished by injecting a sinusoidal low-frequency test current .DELTA.I.multidot.cos (.omega.t) into the laser. The first derivative dV/dI was measured by synchronously detecting the component of the voltage response at frequency .omega.. The second derivative d.sup.2 V/dI.sup.2 was measured similarly by detecting the component of the voltage response at 2.omega.. Higher order derivatives can, in principle, be measured by detecting additional harmonics of the test response. Unfortunately, the unavoidable presence of harmonic distortion in the signal source leads to difficulty in accurately measuring second or higher order derivatives. Furthermore, this prior art derivative determination method requires relatively elaborate instrumentation.
The prior art does know methods for tracking and stabilizing the laser bias at threshold. For instance, D. W. Smith and P. G. Hodgkinson, Proceedings of the 13th Circuits and Systems International Symposium, Houston, 1980, pp. 926-930, describe an optical method that relies on synchronous optical detection of small laser test currents and which, for optimum performance, requires mixing of the low-frequency test signal with the high speed data.
An all-electronic method of threshold detection was disclosed by A. Albanese, Bell System Technical Journal, Volume 57, No. 5, May-June 1978, pp. 1533-1544. This prior art method relies on modeling the laser electrically as two series components, the first being a constant resistance, and the second having a diode-like characteristic, with a junction voltage saturating at laser threshold. Using an operational amplifier, the resistive component is balanced out. An error voltage signal is then generated by measuring the change in junction voltage in response to the data modulation current. When the junction is saturated, the modulation current should induce no change in junction voltage. With sub-threshold bias, however, a detectable change in junction voltage should be observable. The error signal produced by the modulation current can then be used in a feedback circuit to adjust the bias current to a level close to, but below threshold.
The above prior art method requires the detection of the modulation signal by the feedback circuit, necessitating the use of high frequency electronic circuitry. The method is also sensitive to the frequency of occurrence of logic ONE pulses in the data, requires careful tuning out of the laser series resistance, which must track with temperature, and is hampered especially by the fact that many lasers do not show strong junction voltage saturation at threshold.
As shown by the above discussion, prior art methods for stabilizing the bias current of a laser at or near threshold have considerable drawbacks. Due to the importance of reliably stabilizing the bias current of a laser at or near threshold, over a wide range of operating temperature and other operating conditions, a simple and inexpensively implemented method that is substantially free of the drawbacks associated with prior art methods would be of considerable interest. This application discloses such a method. In particular, the method disclosed herein relies on the electrical characteristics of the laser, but does not require saturation of the laser junction voltage, and is insensitive to both laser series resistance and data pattern.